The present invention relates to an apparatus for automatically dimming gas discharge lamps. The apparatus allows gas discharge lamps to be started and then automatically dimmed with no further operator intervention.
A gas discharge lamp basically comprises a tube containing a suitable gas mixture across which is impressed an AC voltage. Gases do not normally conduct electricity since they normally contain neither free electrons nor positive ions. However, if the gas is ionized to some degree, thereby separating electrons from their individual atoms, the resulting free electrons and positive ions may both contribute to the flow of current. The flow of positive ions contributes very little to the total current, however, due to the fact that the positive ions have much greater mass than the electrons and consequently move more slowly under the influence of the same electric field.
When a free electron collides with a neutral gas atom, one of three things may happen: an elastic collision may occur, the atom may be excited above its ground state, or the atom may be ionized releasing one of its electrons. Which of these events occurs depends on the energy of the electron involved in the collision, ionization requiring the highest energy. At each ionizing collision of an electron with a neutral atom, two free electrons are produced in place of one. Successive ionizing collisions, therefore, result in a cumulative buildup in the numbers of free electrons and ions provided that loss is due to recombination and the fusion to the walls of the tube are not excessive. It is the production of charged particles by this cumulative ionization which enables the gas to conduct electricity. The conductivity of the gas is proportional to the ion concentration. Since the ion concentration increases with the total current due to the cumulative ionization, the volt-ampere characteristic of the gas discharge tube has a negative slope. This means that the lamp is not inherently current limiting and for stable operation from a constant voltage supply, the circuit must include a current limiting device such as a resistor. For AC operation, the resistor may be replaced by a reactive impedance such as a transformer or autotransformer which minimizes power losses. Such a resistance or inductive impedance is often called a ballast.
Light is produced by the lamp when collisions between electrons and atoms are insufficient to produce ionization but rather excite electrons to higher energy levels. The energy level may be regarded as a possible orbit for one of the outer electrons of the atom, the highest energy level being equal to the ionization energy when the electron escapes from the atom. An electron and an atom which is raised from the ground state, i.e., the lowest energy level by collision with a free electron, remains in the higher level for a short period and then falls back to the ground state emitting a photon. The resulting light may then be pass through the tube directly or, as in fluorescent lamps, the photons emitted by the gas atoms may excite a phosphor coating on the inside of the tube which then emits light through the process of fluorescence.
Once sufficient cumulative ionization has occurred, a gas discharge lamp will conduct electricity and emit light as long as a sufficient voltage is maintained across the electrodes to accelerate the free electrons within the gas. A continuous supply of electrons is provided by each electrode alternately acting as a cathode during opposite phases of the AC cycle. Upon initial application of the normal operating voltage across the electrodes, however, insufficient numbers of free electrons exist in order to begin the cumulative ionization which carries the lamp current and maintains the discharge. Therefore, the electrodes must first be heated to thermionic emission temperature in order to produce the required free electrons. After conduction through the lamp has begun, it is not necessary for the thermionic emission to continue.
Various methods have been employed to start gas discharge lamps, and, in particular, fluorescent lamps. One method is to use a starter switch SS1 which conducts the initially applied electrical current across both electrodes E1 of the lamp L as shown in FIG. 1. Once the electrodes have been electrically heated to thermionic emission temperature, the switch is opened, and the lamp is started.
Since the switch in the above method must be replaced periodically, starterless circuits have also been devised. In this method, as shown in FIG. 2, AC current is applied to the lamp ballast B1. The ballast may be either a conventional transformer or, as is shown in FIG. 2, an autotransformer. In either case, a portion of the transformer's output is tapped for each electrode E1 in order to produce a current which heats the electrode to thermionic emission temperature.
For reason of both energy efficiency and consumer preference, it is desirable to incorporate dimming devices into fluorescent lamp circuits. Such dimming devices, which are well-known in the art, generally use solid state components such as silicon controlled rectifiers (SCR's) to block portions of each phase of the incoming AC voltage. By only allowing portions of each phase to be conducted to the lamp, the amount of power delivered is thereby reduced. A problem with such dimming devices, when applied to gas discharge lamps, is that full power is needed to start the lamp because of the thermionic emission requirement discussed above. This means that a user must first apply full power to the lamp and then manually adjust the dimming device to the desired level. The dimming device cannot, therefore, be left at the desired setting since it must be readjusted every time the lamp is turned on. Also, when power outages occur, the lamps will not start automatically when power is returned if the dimming device is operative, again requiring manual intervention.
One such dimming device which attempts to deal with the problem of starting gas discharge lamps is that disclosed in U.S. Pat. No. 4,287,455. The '445 device uses power switch SCR's which are gated by a unijunction transistor via a control switch SCR and a pair of optocouplers. In order to effect the appropriate phase control, the power switch SCR's are non-conducting until the unijunction transistor fires a pulse. The unijunction transistor does not fire until a group of capacitors connected to the emitter are charged to the required value. The charging of the capacitors is done by the incoming AC voltage as seen on the secondary side of a transformer and rectifying bridge circuit which is then fed to the capacitors through a potentiometer. Thus, the phase portion of the incoming AC voltage which is conducted to the load depends upon the values of the capacitors and the setting of the potentiometer. In order to provide automatic starting when the device is set to dim the lamp, the capacitors also receive charging current through a charging diode which is also connected to the collector of a starting transistor. Another pair of capacitors are connected to the base of the starting transistor. When that pair of capacitors is charged, the transistor is turned on which reverse biases the charging diode no longer enabling the capacitors connected to the unijunction transistor to be charged via that pathway. Therefore, when power is initially applied to the device, the unijunction transistor capacitors are charged via both the charging diode pathway as well as the potentiometer pathway which causes triggering of the unijunction transistor earlier in the AC cycle than in normal dimming operation. Supposedly, enough power is transmitted to start the lamp before the starting transistor turns on. The capacitors connected to the base of the starting transistor are discharged whenever the power is turned off, thus enabling another starting operation when power is returned.
The device described above, however, suffers from several shortcomings. First, full power is never directly applied to the load during the starting operation. Instead, the power switch SCR's are simply triggered earlier than during normal dimming operation. This means that whether enough power is delivered to start the lamps will depend on the load since, if enough lamps are connected to the device, it is possible that only full power will start the lamps. Second, part of the charging current for the unijunction transistor capacitors always flows through the potentiometer. At a high dim setting, the triggering of the unijunction transistor occurs later in the AC cycle even during the starting operation. Again, depending on the load, the resulting amount of power transmitted may not be enough to start the lamps. Third, the capacitors connected to the starting transistor must discharge enough to turn off the starting transistor before the starting operation can take place. This supposedly happens whenever power is removed from the device, but a power outage of sufficiently short duration would not allow the capacitors to discharge while still extinguishing the lamp. In that case, the lamps would have to be restarted manually.
Therefore, it is an object of the present invention to provide a dimming apparatus, connectable to the ballast of an ordinary gas discharge lamp, which provides an adjustable degree of dimming while allowing the lamp to be started without disturbing the dimming adjustment. It is a further object to provide a dimming apparatus which applies full power to the lamp or lamps upon initial starting with no dependence upon the dimming adjustment.